This invention relates generally to a bearing mounting system for a rotating axle shaft in a final drive assembly and, more particularly, to a bearing mounting system for a rotating axle shaft of a final drive assembly that is capable of accommodating significant radial loads.
Almost every vehicle, including trucks, and farm and construction equipment, includes a final drive assembly that transmits torque from a motor to a driven device or implement. Final drive assemblies of such vehicles typically include inboard final drive assemblies or outboard final drive assemblies. An inboard final drive assembly is located adjacent to the differential. An outboard final drive assembly is typically located adjacent to the spindle and the wheel or other drive unit.
The inboard final drive assembly receives torque from the motor as input and transmits this torque, via, for example, a planetary gear carrier, to a rotating axle shaft. The bearings associated with an inboard final drive assembly are typically located between a stationary housing and the rotating axle shaft. These inboard final drive bearings allow the axle shaft to freely rotate, while at the same time supporting radial and thrust loads associated with external influences exerted on the rotating axle shaft. Thrust loads are those loads that act in a direction parallel to the longitudinal axis of the axle shaft. Radial loads are those loads that act in a direction perpendicular to the longitudinally axis of the axle shaft.
In general, the typical inboard final drive assembly transmits the drive torque from the planetary gear carrier to the axle shaft by spline coupling a rotating member of the planetary gear carrier to the rotating axle shaft. Typically, the axle shaft is directly supported by a pair of bearing assemblies, and thus, externally-applied radial loads experienced by the axle shaft are transmitted to the housing through the bearings. This configuration mitigates radial loads transmitted through the spline coupling. However, because of the limited accessible space on the axle shaft that is available for directly mounting the bearing assemblies to the shaft, this typical configuration results in the bearing assemblies being placed closer together than is optimal, and thus, the bearing assemblies are subjected to higher loads than is desirable.
In an alternative configuration, the separation between the pair of bearing assemblies may be increased by indirectly mounting one of the bearing assemblies to the shaft, i.e., one of the bearing assemblies is no longer mounted directly to the shaft, but rather is mounted to the shaft via an intermediate member, such as a hub connected to a planetary carrier. In this alternative configuration, the spline coupling is detrimentally exposed to the radial loads that would have been reacted by the bearing assembly if it were mounted directly to the axle shaft.
Moreover, during the rotation of the axle shaft, radial loads at the spline interface may not be uniformly distributed especially in view of the planet carrier having some radial movement during operation. When an externally-applied radial load is experienced by the axle shaft, the spline interface may be exposed to substantial alternating compressive and tensile forces as the spline coupling rotates through its 360 degree revolutions.
Thus, one problem with inboard final drive assemblies is how to maximize the distance between the bearing assemblies while at the same time limiting loads transmitted through the spline, thus protecting the spline coupling from damage or premature wear.
Spline couplings inherently include an amount of play or available movement between the splined parts. Implementing a spline coupling may result in significant radial load variance, which is difficult to predetermine, and thus oversized inboard and outboard bearings are necessary. These varying radial loads may be transmitted through the spline coupling in an imprecise manner to the bearing assemblies supporting the rotating axle shaft, thus also reducing the life of these bearing assemblies. Moreover, varying radial loads and spline coupling play introduce misalignment into the planetary gear carrier, which reduces the life of the planetary gear carrier.
External loads applied to the shaft of a conventional inboard final drive assembly are reacted by a pair of bearing assemblies. The greater the axial distance is between the bearing assemblies, the greater is the external load capability of the final drive assembly. Typically, the conventional inboard bearing assembly is not mounted at the very end of the shaft. Accordingly, this limits the distance between the pair of bearing assemblies, thus detrimentally limiting the external load capability of a typical conventional final drive assembly.
In contrast to the inboard final drive assembly previously discussed, the outboard final drive assembly receives torque from a rotating axle shaft and transmits this torque, via, for example, a planetary gear carrier, to the driven device. The bearings associated with an outboard final drive assembly are typically located between a rotating housing, which is attached to the driven device, and a stationary spindle. These outboard final drive bearings allow the driven device to freely rotate. Importantly, however, the spline coupling of the typical outboard final drive assembly is between a stationary spindle and a stationary member of the planetary gear carrier. Since the spline couplings of outboard final drive assemblies are stationary, they are generally not subjected to varying radial loads around all 360 degrees of the spline coupling, as contrasted to the rotating spline couplings of inboard final drive assemblies.
U.S. Pat. No. 4,491,037 to Bullock shows a rotating shaft spline coupled to a rotating member of a planetary gear assembly. A threaded washer is attached to a threaded portion of the rotating shaft and held in place with a threaded nut. The threaded washer is an assembly aid used to hold the planet carrier assembly to the axle group. The rotating axle is supported by the double-tapered bearing assembly, and externally-applied radial loads are reacted by this double-tapered bearing assembly.
Moreover, the inherent clearance and manufacturing tolerance of the threaded washer results in a significant amount of movement or play between the shaft and the threaded washer. Thus, the threaded washer is an unacceptable member for transmitting significant radial loads from the axle shaft to the bearing assembly on the planetary carrier.
There exists a need for a compact final drive assembly rotating axle mounting system that is capable of carrying large and varying loads, while reducing misalignment and premature wear. In particular, there exists a need for an inboard final drive assembly rotating axle mounting system that can be easily configured and withstand significant varying loads in a relatively compact space.
The present invention is directed to overcoming one or more of the problems or disadvantages associated with the prior art.
In one aspect of this invention, a final drive assembly for transmitting torque from a drive source to an output device is provided. The final drive assembly includes a non-rotatable housing, a rotatable shaft, a gear assembly, at least one bearing assembly, and a bushing. The shaft has an outer surface. The gear assembly has a rotatable member coupled to the shaft and is configured to transmit torque from the drive source to the shaft. The bearing assembly is coupled to the housing and configured to rotatably support the rotatable member. The bushing is radially disposed between the rotatable member and the outer surface of the shaft. The bushing, which is configured to transmit substantially all of a radial load transmitted from said shaft to said bearing assembly, is fixedly coupled to one of the rotatable member and the shaft.
In another aspect of the invention, a final drive assembly for transmitting movement from an input device is provided. The final drive assembly includes a stationary housing, a rotatable shaft, a planetary gear assembly, first and second bearing assemblies, and a bushing. The planetary gear assembly has an outer ring gear, a rotatable planetary carrier member, a sun gear, and a plurality of planetary gears. The outer ring gear is fixedly coupled to the stationary housing. The sun gear is configured for being drivingly coupled by the input device. The rotatable planetary carrier member is drivingly coupled to the rotatable shaft. The first bearing assembly is fixedly coupled to the stationary housing and fixedly coupled to the rotatable shaft. The first bearing assembly rotatably supports the rotatable shaft relative to the stationary housing. The second bearing assembly is axially displaced relative to the first bearing assembly. Moreover, the second bearing assembly is fixedly coupled to the stationary housing and fixedly coupled to the rotatable planetary carrier member. The second bearing assembly rotatably supports the rotatable shaft relative to the stationary housing. The bushing is radially disposed between the rotatable planetary carrier member and the rotatable shaft. The planetary carrier member is aligned relative to the rotatable shaft through the bushing.
In yet another aspect of the invention, a method for transmitting movement of an input source to a rotatable output member through a final drive assembly, which is subjected to an external load applied to the output member, is provided. The final drive assembly includes a planetary gear assembly supported within a housing. The method includes supporting the output member through a first bearing assembly, supporting a planetary carrier member of the planetary mechanism through a second bearing assembly, and transferring a portion of the external load to the housing through the planetary carrier member of the planetary gear assembly. Substantially all of the portion of the load being transferred between the planetary carrier member and the output member is directed through a bushing disposed between the planetary carrier member and the output member.